On Sunday night Nov. 13-14, the full moon will be the closest and biggest one since 1948. We call it a supermoon! Credit: Göran Strand / astrofotografen.se

Get ready! We’ve got a big moon on the rise Sunday night, the biggest and closest since January 26, 1948.

The moon is full every month, but not all full moons are equal in distance, size and brightness. If the moon’s orbit around the Earth were a perfect circle, they would be. But the moon’s orbit is an imperfect or “squashed” circle called an ellipse. At one end of the ellipse, called perigee, the moon is closest to the Earth. At the other end, called apogee, farthest. It passes the perigee and apogee points once every 27.5 days, the time it takes the moon to orbit the Earth.

The moon’s orbit around Earth is an ellipse with one end closer to the planet (perigee) and the other farther (apogee). A supermoon occurs when the full moon is close to perigee. Not to scale. Credit: Bob King

In exactly the same way that some people are close-talkers and get right in your face when speaking to you, they loom large. You might ask them to back off a bit. The moon at perigee is like that. Its average distance from Earth is around 239,000 miles, but at perigee, it’s more like 225,000 miles. Being 14,000 miles closer means the moon looks bigger. Brighter too! At apogee, the moon backs off to about 252,000 miles and looks both smaller and fainter.

A supermoon occurs when the moon happens to be full while at the same time near perigee or closest to the Earth. A perigee or supermoon is on average 7% bigger and 16% brighter than an average full moon, but during an unusually close perigee, the full moon can be 12–14% larger than a full moon at apogee and 30% brighter. Those differences are significant but seeing them is another thing. After all, we can’t drag a distant full moon and plunk it next to a supermoon, right? We either have to recall the average appearance of a full moon and compare in our mind’s eye or find a way to measure the difference.

My unscientific but perhaps practical Supermoon Sighter made with an index card and pair of scissors. Credit: Bob King

That’s why I cooked up a little device — the Supermoon Sighter — anyone can make to at least provide a rough measure of the moon’s apparent size using just the naked eye. With a pair of scissors, cut a series a slots of varying widths in an index card then hold the card as best you can parallel to your face and at maximum arm length while facing the noon. Close one eye and use the other to determine the slot into which the Moon fits snugly. Be sure to ink the date and time under that slot.

Tuck the card somewhere where you’ll remember it and then produce it again for the apogee Full Moon of June 8, 2017, when the moon will be significantly smaller. Repeat the procedure and see what slot the moon fits into this time. When conducting your experiment, you’ll find that it’s easier to see the slots if the moon is viewed in a twilit sky.

Apogee full moons are sometimes called mini or micro moons, and as you’ve guessed by now, no one celebrates them. Like TVs, we prefer our full moons big. Supermoons occur several times a year, but they’re also not all equal. Some are closer than others. If the full moon occurs within a few hours of perigee, that supermoon will be a bit closer, bigger and brighter than a supermoon occurring plus or minus a day of perigee.

Two other things come into play to make this Sunday night’s supermoon even closer than normal, the closest actually since January 26, 1948. When a full moon arrives at its perigee point, it makes a neat lineup with two gravitational heavyweights, the Earth and sun. The sun in particular holds sway over the moon; despite its much greater distance, it’s far more massive than the Earth and its gravitation influence about twice as strong.

When the full moon comes up Sunday night, be sure to look for Earth’s shadow to rise at the same time. It’s the dark, purplish-gray band in the photo above. This is the shadow of our planet cast across the atmosphere. Coolest! Also watch for distortions in the moon’s shape due to atmospheric turbulence. Use binoculars for the best view. Credit: Bob King

But I digress. The lineup of the three bodies shifts the moon’s orbit slightly in the Earth-sun direction, moving the perigee point a bit closer to Earth. Finally, if a supermoon occurs from late fall through mid-winter, when Earth is closest to the sun, that extra bit of solar tug will shift lunar perigee even closer. All these things are happening at the same time during November’s full moon conspire to bring Sunday night’s full moon just 221,524 miles from Earth or about 4,000 miles closer (half an Earth diameter) than a typical perigee. Truly, this moon will be a super duper supermoon!

If a new or full Moon lies at the apogee or perigee point, the shape of the Moon’s orbit gets slightly altered, resulting in a closer perigee than normal and an exceptional supermoon. Not to scale.Bob King

The moment of closest approach for the November full moon occurs on Monday morning at 7:52 a.m. Central Standard Time (8:52 a.m. Eastern, 6:52 a.m. Mountain and 5:52 a.m. Pacific), too late for skywatchers to see in the eastern half of the U.S. as the moon will have set by that time. But folks living in the western U.S., Alaska and Hawaii will behold the moon at its largest. Most of us instead will gather to view the moonrise Sunday evening some hours before closest approach, when it will be a tiny, tiny bit farther, but not enough to make a difference to the eye.

Find the time of moonrise for your location here, face east and look for the big moon to mount the horizon within a few minutes of sunset. Whenever the moon is in full phase, it lies directly opposite the sun in the sky and rises around sundown.

For observers in Europe, Africa and parts of Asia, the closest-moon moonrise will occur on Monday evening the 14th. For all of us watching, the Moon Illusion will further magnify the moon’s jumbo appearance. At moonrise, our brains judge the full moon as much bigger than it really is because we unconsciously compare it to familiar surroundings. In reality, the rising moon is a tad farther away and smaller than when it’s overhead. When near the horizon, we have to look across the curvature of the Earth and then out to the moon. When overhead, there’s no Earth in the way and we’re a little closer to the moon. The difference is small, only 1.5%, but it’s real.

Bad weather forecast for Sunday night? Click on the photo to view it Monday morning on the Virtual Telescope site. Credit: Gianluca Masi

The moon will be in the constellation Aries the Ramon Sunday night and stand high above the horizon around midnight when it’s due south. These high-flying full moons tell us that winter is near. Come December and January, the full moon shines down from its highest perch in the sky in constellations Taurus and Gemini, the very same ones the sun called home back in June and July!

If it turns out to be cloudy at your site, you can watch the supermoon via live webcam at Italian astronomer Gianluca Masi’s Virtual Telescope Project starting at 16:00 UT (10 a.m. Central time) November 14th. You can also watch it live on Slooh’s website where they’re calling it the Mega Beaver Moon because the November full moon’s traditional name is the Beaver Moon. The colonists and eastern Native American tribes set traps for beavers then in order to have a good supply of furs for the winter ahead.

We have to cool our heels a while before the next closer supermoon — that one happens on Nov. 25, 2034, when the moon will be 221,485 miles away and just a tiny bit larger.

Happy moongazing!

*** If you’d like to learn more about the moon, I’ve got it all in my new book, Night Sky with the Naked Eye,that just published this week. Just click one of the photos below to go to the site of your choice — Amazon, Barnes & Noble or Indiebound. The book is also available locally at Barnes & Noble in Duluth (and at many BNs) and The Bookstore at Fitger’s.